Support OIDC backchannel logout

[morki]

https://openid.net/specs/openid-connect-backchannel-1_0.html

That would be awesome, we use this in Keycloak now. I think the front channel logout is not required anymore, because it is not working nowaday in modern browsers I think.

[sebadob]

I think the front channel logout is not required anymore, because it is not working nowaday in modern browsers I think.

That a myth and just PR nonsense made up mostly by google. There is absolutely no issue with front channel logout and there never will be.

I may implement backchannel logout at some point, but I have other more important things on the TODO currently.
However, a PR is always welcome.

[avdb13]

https://openid.net/specs/openid-connect-backchannel-1_0.html

It's quite straightforward, might have an attempt at this as I tried to implement backchannel logout for Matrix in the past.

[sebadob]

I decided to add the backchannel logout after the UI migration. Also, in both directions, in case a client supports it. This makes a lot of sense in combination with SCIM in the future to log out users' independent client sessions.